Dose Course 2018_Flipping book

7

How to get the dose distribution?

❑ To get a depth dependence one has to correct the planar fluence of electrons

D ( r , z ) = d p

( r , z ) × g ( z )

❑ The weighting factor g ( z ) is determined such that the dose as a function of depth on the central axis for a given field size exactly equals the measured central axis depth dose o Corrected to infinite SSD o Bremsstrahlung dose, is subtracted (assumed constant at all depths less than R p

D measured , electrons _ only , SSD =¥

(0,0, z )

❑ In practice, g ( z ) must be determined for a range of field sizes for a beam of a particular energy as the model can only predict the change over a small range of field sizes

From Handbook of Radiotherapy Physics Ed Mayles, Nahum and Rosenwald

Dublin 2018